Apparatus and method for detecting ntsc co-channel interference

ABSTRACT

A television set includes an ATSC (Advanced Television Systems Committee) receiver, which includes an NTSC (National Television Systems Committee) co-channel interference detector based on carrier tracking of the NTSC video carrier signal. The NTSC co-channel interference detector includes a carrier tracking loop and a decision device. The carrier tracking loop processes a received signal for detecting the possible presence of the NTSC video carrier signal and for providing a tracking signal representative thereof. The decision device receives the tracking signal and recovers a DC offset therefrom. The decision device then determines that NTSC co-channel interference is present if the DC offset signal is greater than a predefined threshold.

BACKGROUND OF THE INVENTION

The present invention generally relates to communications systems and,more particularly, to an interference detector in a receiver.

During the transition from analog to digital terrestrial television inthe United States, both analog NTSC (National Television SystemsCommittee) based transmissions and digital ATSC-HDTV (AdvancedTelevision Systems Committee-High Definition Television) basedtransmissions are expected to co-exist for a number of years. As such,an NTSC broadcast signal and an ATSC broadcast signal may share the same6 MHz wide (millions of hertz) channel. This is illustrated in FIG. 1,which shows the relative spectral positions of the NTSC signal carriers(video, audio and chroma) with respect to the digital VSB (VestigialSideband) ATSC signal spectrum. Thus, an ATSC receiver must be able toefficiently detect and reject NTSC co-channel interference.

In an ATSC-HDTV digital receiver, NTSC co-channel interference rejectionmay be performed by the comb filter (e.g., see, United States AdvancedTelevision Systems Committee, “ATSC Digital Television Standard”,Document A/53, Sep. 16, 1995). The comb filter is a 12 symbol linearfeed-forward filter with spectral nulls at or near the NTSC signalcarriers, and is only applied when NTSC interference is detected (e.g.,see, United States Advanced Television Systems Committee, “Guide to theUse of the ATSC Digital Television Standard”, Document A/54, Oct. 4,1995). Tests have shown that the comb filter performs efficient NTSCsignal rejection for D/U (Desired-to-Undesired) signal power ratios upto 16 dB (decibels). The D/U signal power ratio is defined as theaverage digital VSB ATSC signal power divided by the average NTSC peaksignal power.

Since the comb filter is only applied when NTSC interference isdetected, it is necessary to first detect the presence of NTSCco-channel interference. Further, it is desirable to be able to detectthe NTSC co-channel interference in high D/U ratios. The above-mentioned“Guide to the Use of the ATSC Digital Television Standard,” describes animplementation of an NTSC detector that uses the power differencebetween the input signal and the output signal of the comb filter. Inparticular, this implementation detects that an NTSC co-channel signalis present when there is a substantial difference in power between theinput signal and the output signal of the comb filter. Unfortunately,this design is not reliable for D/U ratios above 10 dB.

SUMMARY OF THE INVENTION

In accordance with the principles of the invention, a co-channelinterference detector includes a carrier tracking loop for processing areceived signal to provide a tracking signal indicative of a possiblepresence of at least one carrier of an interfering signal and a decisiondevice for determining if the interfering signal is present as afunction of the tracking signal.

In an embodiment of the invention, a television set includes an ATSCreceiver, which includes an NTSC co-channel interference detector basedon carrier tracking of the NTSC video carrier signal. The NTSCco-channel interference detector includes a carrier tracking loop and adecision device. The carrier tracking loop processes a received signalfor detecting the possible presence of the NTSC video carrier signal andfor providing a tracking signal representative thereof. The decisiondevice receives the tracking signal and recovers a DC offset therefrom.The decision device then determines that NTSC co-channel interference ispresent if the DC offset signal is greater than a predefined threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a comparison of an NTSC signal spectrum and a ATSC signalspectrum;

FIG. 2 shows an illustrative high-level block diagram of a TV setembodying the principles of the invention;

FIG. 3 shows a portion of a receiver embodying the principles of theinvention;

FIG. 4 shows an illustrative carrier tracking loop for use in thereceiver of FIG. 3;

FIG. 5 shows an illustrative method in accordance with the principles ofthe invention;

FIG. 6 shows an illustrative simulator configuration;

FIGS. 7 and 8 show illustrative simulation results; and

FIGS. 9-10 show other embodiments in accordance with the principles ofthe invention.

DETAILED DESCRIPTION

Other than the inventive concept, the elements shown in the figures arewell known and will not be described in detail. For example, other thanthe inventive concept, a television, and the components thereof, such asa front-end, Hilbert filter, carrier tracking loop, video processor,remote control, etc., are well known and not described in detail herein.In addition, the inventive concept may be implemented using conventionalprogramming techniques, which, as such, will not be described herein.Finally, like-numbers on the figures represent similar elements.

A high-level block diagram of an illustrative television set 10 inaccordance with the principles of the invention is shown in FIG. 2.Television (TV) set 10 includes a receiver 15 and a display 20.Illustratively, receiver 15 is an ATSC-compatible receiver. It should benoted that receiver 15 may also be NTSC-compatible, i.e., have an NTSCmode of operation and an ATSC mode of operation such that TV set 10 iscapable of displaying video content from an NTSC broadcast or an ATSCbroadcast. However, in the context of this description, the ATSC mode ofoperation is described. Receiver 15 receives a broadcast signal 11(e.g., via an antenna (not shown)) for processing to recover therefrom,e.g., an HDTV video signal for application to display 20 for viewingvideo content thereon. As noted above, and shown in FIG. 1, signal 11may include not only a broadcast ATSC signal but also interference froma co-channel broadcast NTSC signal. In this regard, receiver 15 of FIG.2 includes a rejection filter (not shown), such as the above-mentionedcomb filter, for removing the NTSC signal interference as describedabove and, in accordance with the principles of the invention, alsoincludes an NTSC co-channel interference detector based on carriertracking of the NTSC video carrier signal. As described further below,upon detection of NTSC co-channel interference, the NTSC co-channelinterference detector enables use of the rejection filter for theprocessing of signal 11 for mitigation of the NTSC co-channelinterference.

Turning now to FIG. 3, that relevant portion of receiver 15 including anNTSC co-channel interference detector in accordance with the principlesof the invention is shown. In particular, receiver 15 includesanalog-to-digital converter (ADC) 105, automatic gain control (AGC) 110,band-pass filter (BPF) 115, delay/Hilbert filter element 120, carriertracking loop (CTL) 125, averaging (avg.) filter 135 and comparator 140.

Input signal 101 represents a digital VSB modulated signal in accordancewith the above-mentioned “ATSC Digital Television Standard” and iscentered at a specific IF (Intermediate Frequency) of f_(IF) Hertz.However, as also noted above, input signal 101 may also contain NTSCco-channel interference. Input signal 101 is sampled by ADC 105 forconversion to a sampled signal, which is then gain controlled by AGC110. The latter is noncoherent and is a mixed mode (analog and digital)loop that provides a first level of gain control (prior to carriertracking), symbol timing and sync detection of the VSB signal includedwithin signal 101. AGC 110 basically compares the absolute values of thesampled signal from ADC 105 against a predetermined threshold,accumulates the error and feeds that information, via signal 112, backto the tuner (not shown) for gain control prior to ADC 105. As such, AGC110 provides a gain controlled signal 113 to ATSC VSB processingcircuitry (not shown) and to BPF 115. In accordance with a feature ofthe invention, BPF 115 is centered at the NTSC video carrier and has anarrow bandwidth less than or equal to 600 KHz (thousands of hertz).Assuming no transmitted offsets between the VSB signal and a co-channelNTSC signal, and assuming high side injection, the NTSC video carrier isexpected to be at a frequency, f_(VIDEO), where f_(VIDEO)=f_(IF)−1.75MHz.

The output signal from BPF 115 is then passed through delay/Hilbertfilter element 120. The latter includes a Hilbert filter and anequivalent delay line that matches the Hilbert filter processing delay.As known in the art, a Hilbert Filter is an all-pass filter thatintroduces a −90° phase shift to all input frequencies greater than 0(and a +90° degree phase shift to negative frequencies). The Hilbertfilter allows recovery of the quadrature component of the output signalfrom BPF 115. In order for the CTL to correct the phase and lock to theNTSC video carrier both the in-phase and quadrature components of thesignal are needed.

The output signal 121 from delay/Hilbert filter element 120 is a complexsample stream comprising in-phase (I) and quadrature (Q) components. Itshould be noted that complex signal paths are shown as double lines inthe figures. Signal 121 is applied to carrier tracking loop (CTL) 125,which is a phase locked loop that processes the complex sample stream ofsignal 121 to down convert the IF signal to baseband and correct forfrequency offsets between the transmitter (not shown) of the broadcastNTSC video carrier and the receiver tuner Local Oscillator (not shown).CTL 125 is a second order loop, which, in theory, allows for frequencyoffsets to be tracked with no phase error. In practice, phase error is afunction of the loop bandwidth, input phase noise, thermal noise andimplementation constraints like bit size of the data, integrators andgain multipliers.

Turning for the moment to FIG. 4, an illustrative embodiment of CTL 125is shown. CTL 125 includes complex multiplier 150, phase detector 155,loop filter 160, combiner (or adder) 165, numerically controlledoscillator (NCO) 170 and sine/cosine (sin/cos) table 175. It should benoted that other carrier tracking loop designs are possible, as long asthey achieve the same performance. Complex multiplier 150 receives thecomplex sample stream of signal 121 and performs de-rotation of thecomplex sample stream by a calculated phase angle. In particular, thein-phase and quadrature components of signal 121 are rotated by a phase.The latter is provided by signal 176, which represents particular sineand cosine values provided by sin/cos table 175 (described below). Theoutput signal from complex multiplier 150, and for that matter CTL 125,is signal 126, which represents a de-rotated complex sample stream.Signal 126 is also referred to herein as the tracking signal. As can beobserved from FIG. 4, tracking signal 126 is also applied to phasedetector 155, which computes any phase offset still present in thetracking signal 126 and provides a phase offset signal indicativethereof. This computation can be performed with a “I*Q” or a “sign(I)*Q”function. The phase offset signal provided by phase detector 155 isapplied to loop filter 160, which is a first order filter withproportional-plus-integral gains. Ignoring for the moment combiner 165,the loop filtered output signal from loop filter 160 is applied to NCO170. The latter is an integrator, which takes as an input signal afrequency, and provides an output signal representative of phase anglesassociated with the input frequency. However, in order to increase theacquisition speed, the NCO is fed a frequency offset, F_(OFFSET),corresponding to f_(VIDEO), which is added to the loop filter outputsignal via combiner 165 to provide a combined signal to NCO 170. NCO 170provides an output phase angle signal 171 to sin/cos table 175, whichprovides the associated sine and cosine values to complex multiplier 150for de-rotation of the CTL input signal 121 to provide tracking signal126.

Returning now to FIG. 3, when NTSC co-channel interference is present inthe received signal 11 of FIG. 2, tracking signal 126 includes a DCoffset at baseband that is proportional to the power level of thereceived NTSC video carrier. The sign of this DC offset depends onpossible 180° ambiguity of the carrier tracking loop, which depends onthe carrier tracking loop implementation. As shown in FIG. 3, thein-phase (real) component of tracking signal 126 is applied to avg.filter 135, which is a low pass filter (LPF) with low bandwidth, e.g.,100 Hz (Hertz). In accordance with a feature of the invention, avg.filter 135 averages the in-phase component of tracking signal 126 in thedigital domain to provide signal 136, which represents the DC offset,i.e., an average DC measure of the possible presence of NTSC co-channelinterference. As such, signal 136 should be independent of the presenceof additive white Gaussian noise (AWGN), phase noise and the VSB signal,since they average out to zero. It should be noted that if the NTSCvideo carrier is not present in the received signal 11, or if the NTSCvideo carrier power level is too low such that CTL 125 does notconverge, the average DC value of signal 136 is zero.

A determination of the presence of NTSC co-channel interference isperformed by comparator 140, which provides output flag signal 141.Reference at this time should also be made to FIG. 5, which shows anillustrative flow chart in accordance with the principles of theinvention. In particular, signal 136, which also represents the possibledetection of the NTSC video carrier signal, is applied to comparator 140(step 305, FIG. 5). The latter compares the value of signal 136 to apredetermined positive threshold value (step 310, FIG. 5). If theabsolute value of the DC offset is greater than the predeterminedthreshold, comparator 140 sets output flag signal 141 to a predeterminedvalue, e.g., a value associated with a logical “ONE,” which representsdetection of the NTSC video carrier, i.e., detection of co-channelinterference. The output flag signal is then used by other circuitry(not shown) to enable suitable NTSC rejection filtering, as describedearlier (step 320, FIG. 5). However, if the absolute value of the DCoffset is not greater than the predetermined threshold, then output flagsignal 141 is set to a value associated with a logical “ZERO,” whichrepresents no co-channel interference. In this case, the output flagsignal may also be used to disable NTSC rejection filtering if such waspreviously enabled (step 315, FIG. 5). Thus, and in accordance with theprinciples of the invention, if a signal indicative of the NTSC videocarrier is detected, then receiver 15 of TV set 10 provides for suitableNTSC rejection filtering, e.g., via a comb filter as described above.

A simulation of the above described embodiment subsequent to ADC 110 wasperformed by a program written in the C programming language. Thisprogram processed different input data files, each data filerepresenting an ATSC signal with added NTSC co-channel interference at aspecific D/U ratio. The different data files were obtained using thesimulator configuration shown in FIG. 6. An NTSC video source 420provides an NTSC signal to NTSC modulator 425. The latter provides anNTSC transmission signal carrying the NTSC video to attenuator 430,which is used to vary the power level of the NTSC transmission signalfor obtaining different D/U ratios. The attenuated NTSC transmissionsignal is provided to combiner 410, which adds the attenuated NTSCtransmission signal to an ATSC signal provided by VSB modulator 405,which was set to a Pseudo-Noise (PN) mode. Both the NTSC transmissionsignal and the ATSC signal are centered at an IF of 44 MHz. The combinedsignal—the ATSC signal along with the co-channel NTSC interferencesignal—is applied to a digital oscilloscope 415. In particular, digitaloscilloscope 415 samples the combined signal at a sampling rate off_(SAMP)=25 Msamples/sec (millions of samples per second) and 8bits/sample, which results in a sampled high-injection signal centeredat an IF of f_(IF)=6 MHz. Other sampling rates and intermediatefrequencies could also be used.

Some results of this simulation are shown in FIGS. 7 and 8. FIG. 7 showsillustrative graphs for a D/U ratio of 10 dB. In particular, FIG. 7shows graphs of CTL frequency deviation and DC offset at the averagingfilter output versus the number of samples. Both curves show a stableconvergence of the carrier tracking loop and the DC offset,respectively.

Turning now to FIG. 8, table 1 shows the simulation results for severalD/U ratios, specifying the corresponding absolute value of the DC offsetat the output of the averaging filter and the carrier tracking loopconvergence status. From table 1, it can be observed that the carriertracking loop efficiently detects co-channel NTSC interference up to aD/U of about 20 dB. The entries of table 1 can also be used to provideillustrative threshold values for use in comparator 140 of FIG. 3. Forexample, if a threshold value is set at 31, then the decision device(e.g., comparator 140 of FIG. 3) identifies the presence of NTSC for D/Ubelow 18 dB.

In accordance with a feature of the invention, the entries of table 1(or similar entries) can also be stored a priori in a memory (not shown)of receiver 15 to provide an estimate of the corresponding D/U ratio fora particular DC offset. One such illustrative embodiment is shown inFIG. 9, which is similar to the embodiment shown in FIG. 3 except thatcomparator 140 includes a look-up-table (LUT) 143 and also provides anoutput signal 142. The latter is an estimate of the corresponding D/Uratio for a particular value of DC offset provided by signal 136 and, assuch, is an indication of how bad the NTSC interference is.Illustratively, LUT 143 of comparator 140 stores a table correspondingto, e.g., the “D/U (dB)” and “DC offset” columns of table 1 of FIG. 8.Comparator 140 quantizes the DC offset represented by signal 136 intoone of the predefined DC offsets of Table 1 and provides thecorresponding D/U ratio entry as an estimate of the D/U ratio on signal142.

As described above, an NTSC co-channel interference detector is based oncarrier tracking of the NTSC video carrier. Such a co-channelinterference detector is able to efficiently detect a co-channel NTSCsignal up to the very high D/U ratio of about 20 dB. It should be notedthat this same detector can also be employed to track the NTSC audio orchroma carrier, although it is not expected to be as efficient, due tothe smaller power of these carriers compared to the video carrier.However, these alternate detectors would be useful for special caseswhen multipath propagation in the terrestrial channel produces aspectral null on the NTSC video carrier frequency, affecting itsdetection, but leaving the other carriers intact.

Although the inventive concept was described above in the context of atelevision receiver and NTSC co-channel interference, the inventiveconcept is not so limited and applies to any receiver that operates inthe presence of one or more co-channel interfering signals. Turning nowto FIG. 10, another embodiment in accordance with the principles of theinvention is shown. A co-channel interference detector 40 includesinterference carrier tracking element 50 and decision device 55. Areceived signal 49 is applied to interference carrier tracking element50, which provides an output signal 51 proportional to the possiblepresence of at least one carrier (an interfering carrier) of aninterfering signal present in received signal 49. Decision device 55receives the output signal 51 and determines if the interfering signalis present as a function of a parameter of output signal 51, e.g.,voltage level, frequency, phase, etc. Decision device 55 may alsofurther process output signal 51 before determining whether theinterfering signal is present. For example, and as described above withrespect to FIG. 3, decision device 55 may first perform an averaging ofoutput signal 51. Alternatively, or in addition to, decision device 55may also calculate other statistical parameters, such as the standarddeviation, etc. Illustratively, carrier tracking element 50 and decisiondevice 55 may include elements similar to those shown and describedabove with respect to FIGS. 3 and 4 but are not so limited. It shouldalso be noted that groupings of components for particular elementsdescribed and shown herein are merely illustrative. For example,although FIG. 3 shows a Hilbert filter external to the carrier trackingloop, this is not required and, e.g., the Hilbert filter could have beenshown and described as a part of the carrier tracking loop.

As such, the foregoing merely illustrates the principles of theinvention and it will thus be appreciated that those skilled in the artwill be able to devise numerous alternative arrangements which, althoughnot explicitly described herein, embody the principles of the inventionand are within its spirit and scope. For example, although illustratedin the context of separate functional elements, these functionalelements may be embodied on one or more integrated circuits (ICs).Similarly, although shown as separate elements, any or all of theelements may be implemented in a stored-program-controlled processor,e.g., a digital signal processor. Further, although shown as elementsbundled within TV set 10, the elements therein may be distributed indifferent units in any combination thereof. For example, receiver 15 maybe a part of a device, or box, physically separate from the device, orbox, incorporating display 20, etc. It is therefore to be understoodthat numerous modifications may be made to the illustrative embodimentsand that other arrangements may be devised without departing from thespirit and scope of the present invention as defined by the appendedclaims.

1. Apparatus for use in detecting an interfering signal, the apparatuscomprising: a carrier tracking loop for processing a received signal toprovide a tracking signal indicative of a possible presence of at leastone carrier of an interfering signal; and a decision device fordetermining if the interfering signal is present as a function of thetracking signal.
 2. The apparatus of claim 1, wherein the decisiondevice further comprises: a filter for averaging the tracking signal;and a comparator for comparing the averaged tracking signal to apredetermined threshold for determining if the interfering signal ispresent.
 3. The apparatus of claim 2, further comprising a look-up-tablefor providing a D/U (Desired-to-Undesired) signal power ratio from theaveraged tracking signal.
 4. The apparatus of claim 3, wherein the D/Usignal power ratio is defined as an average digital vestigial sidebandATSC (Advanced Television Systems Committee) signal power divided by anaverage NTSC (Nation Television System Committee) peak signal power. 5.The apparatus of claim 1, wherein the interfering signal is an NTSC(National Television Systems Committee) signal and the at least onecarrier is an NTSC video carrier.
 6. The apparatus of claim 1, whereinthe interfering signal is an NTSC signal and the at least one carrier isan NTSC chroma carrier.
 7. The apparatus of claim 1, wherein theinterfering signal is an NTSC signal and the at least one carrier is anNTSC audio carrier.
 8. The apparatus of claim 1, wherein the carriertracking loop further comprises: a bandpass filter for filtering thereceived signal to provide a bandpass filtered signal, wherein thebandpass filter is centered about the at least one carrier; adelay/Hilbert filter for filtering the bandpass filtered signal toprovide a complex signal; and a carrier tracking loop operative on thecomplex signal for providing the tracking signal.
 9. The apparatus ofclaim 8, wherein the carrier tracking loop further comprises: a phasedetector for processing the tracking signal to provide a phase offsetsignal indicative of a phase offset in the tracking signal; a loopfilter for filtering the phase offset signal; a combiner for combiningthe loop filtered phase offset signal and a frequency offset signal toprovide a combined signal, where the frequency offset signal is equal toa frequency of the at least one carrier signal; a numerically controlledoscillator responsive to the combined signal for providing a phase anglesignal; a sine/cosine table for receiving the phase angle signal and forproviding associated sine and cosine values; and a complex multiplierfor multiplying the complex signal by the sine and cosine valuesprovided by the sine/cosine table to provide the tracking signal.
 10. Atelevision set comprising: a receiver for receiving a broadcast ATSC(Advanced Television Systems Committee) compatible signal includingvideo content; and a display for viewing the video content; wherein thereceiver further comprises an NTSC (National Television SystemsCommittee) co-channel interference detector based on carrier tracking ofat least one carrier of an NTSC signal.
 11. The television set of claim10, wherein the at least one carrier is an NTSC video carrier.
 12. Theapparatus of claim 10, wherein the at least one carrier is an NTSCchroma carrier.
 13. The apparatus of claim 10, wherein the interferingsignal is an NTSC signal and the at least one carrier is an NTSC audiocarrier.
 14. A method for use in detecting an interfering signal, themethod comprising: processing a received signal to provide a trackingsignal indicative of a possible presence of at least one carrier of aninterfering signal; and determining if the interfering signal is presentas a function of the tracking signal.
 15. The method of claim 14,wherein the determining step includes the steps of: averaging thetracking signal; and comparing the averaged tracking signal to apredetermined threshold for determining if the interfering signal ispresent.
 16. The method of claim 15, further comprising the step ofproviding a D/U (Desired-to-Undesired) signal power ratio from theaveraged tracking signal.
 17. The method of claim 16, wherein the D/Usignal power ratio is defined as an average digital vestigial sidebandATSC (Advanced Television Systems Committee) signal power divided by anaverage NTSC (Nation Television System Committee) peak signal power. 18.The method of claim 14, wherein the interfering signal is an NTSC(National Television Systems Committee) signal and the at least onecarrier is an NTSC video carrier.
 19. The method of claim 14, whereinthe interfering signal is an NTSC signal and the at least one carrier isan NTSC chroma carrier.
 20. The method of claim 14, wherein theinterfering signal is an NTSC signal and the at least one carrier is anNTSC audio carrier.
 21. The method of claim 14, wherein the processingstep includes the steps of: filtering the received signal to provide abandpass filtered signal, wherein the bandpass filtered signal has abandwidth centered about the at least one carrier; filtering thebandpass filtered signal to provide a complex signal; and processing thecomplex signal with a carrier tracking loop for providing the trackingsignal.
 22. The method of claim 21, wherein the processing the complexsignal step includes the steps of: processing the tracking signal toprovide a phase offset signal indicative of a phase offset in thetracking signal; filtering the phase offset signal to provide a filteredphase offset signal; combining the filtered phase offset signal and afrequency offset signal to provide a combined signal, where thefrequency offset signal is equal to a frequency of the at least onecarrier signal; processing the combined signal to provide a phase anglesignal proportional to the frequency of the combined signal; providingsine and cosine values associated with the provided phase angle; andmultiplying the complex signal by the provided sine and cosine values toprovide the tracking signal.
 23. The method of claim 14, furthercomprising the step of enabling a rejection filter for processing thereceived signal to remove the interfering signal when the interferingsignal is determined to be present.